Troubleshooting a brake problem without much success Left brake much softer than the right. Tried bleeding several times, pedal good for several applications then back to same. Pedal pumps up to normal and holds without leaking down.
Decided to rebuild the master cylinder. The lock o seal oring looked pretty bad. No visible damage to cylinder. Rebuilt in accordance with service manual. Bled system, worked great for about 20 applications. Now back to soft pedal.
Is it time to replace the cylinder? Have I missed something else?
Are you testing "static" (i.e., without the airplane moving)?
Also, is there any indication of leakage around the caliper?
With a friend, we pulled our hair out on a brake problem like
this (on an early C-180) until we discovered the wheel halve
bolts were loose (!). The whole thing was moving around
enough to push the caliper pistons in & out thereby pumping
air into the system.
I also had an ever so slight leak on a double-puck caliper
that just wouldn't hold pressure. I tried dressing the bores,
replacing the O-rings, but could never get it to hold pressure.
Replaced the caliper with another known / good one, and presto....
problem solved.
Have you been back bleeding from the bottom up? Thats the only way I've ever been able to get all the air out. I use a Plews pump type oil can with a one foot length of tight fitting clear vinyl plastic tubing attached to the spout. This will fit tight over the bleeder valve and will allow you to see that there are no air bubbles being pumped in.
Bleeding from the bottom up like BL describes is the only way to go. Sometimes after doing this, you might still have a little soft pedal action, but then usually by the next day it's OK. Just a few tiny bubbles that have to kinda migrate up to the reservior on their own, I guess.
We are bleeding from the bottom up using the technique you described. I'll try it again to see if some air did move up. How much fluid do you usually pump through the system?
Get a 1/8 pipe barbed fitting and a chunk of clear hose. Put the hose in a glass jar by the rudder pedals and hook the barb fitting up to the master you're bleeding. Have a buddy watch until there are no bubbles. Do not pump too much as to run out of fluid and introduce air. Most pumpers can be kept topped up during the process with a little care if you only pump half the capacity or so.
I just helped do this on a Twin Commanche the other day. Pretty nice job on that airplane, as the reservoir is mounted on the firewall & is real easy to look into. The lines had been left off during caliper repair & so lots of fluid had leaked out, and air in. We pumped about 1-1/2 pumper-cans up into the system. We started out with the reservoir about half full, we stopped when it was up close to where it should be. We then tested the pedal action, and it was fine.
I've done the same with my 170, but it's a lot harder to check the reservoir level mounted at the pedals like they are. Inevitably a mess of fluid ends up in the belly around the pedal area. If I fill the reservoirs up to where I think they should be, fluid comes out the breather holes in the caps and ends up coming out a drain hole &running back on the belly. I don't believe the 5606 fluid is corrosive, but it sure is messy.
Are you sure the caliper and the brake disc match? On my current plane when I got it, had similar problems, the discs were too thin for the calipers, Installed thicker discs to match the calipers and solved the problem.
If one gets careless and allows the hyd fluid level in the oil-can to drop too far, it will introduce air into the system. So check it often.
A clear hose slipped onto the plastic vent and leading back out of the cockpit can help in preventing fluid spillage into the belly. (Otherwise enjoy the anti-corrsive effects of 5606.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
Another thing to check that happened to us. My father put the wheel back on after doing the wheel bearings. He didn't get it seated on the bearings but instead it hung up on the dust seal washers. The resultant loose wheel gave us intermittant Lt brake that we could pump up. When installed properly the problem went away. Just a suggestion to add to your list. Brian
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Left brake much softer than the right. Tried bleeding several times, pedal good for several applications then back to same. Pedal pumps up to normal and holds without leaking down. 
